Nucleation and early growth during solidification of aluminum-copper alloys

Abstract

Using a relatively simple experimental setup, thermal history, microstructural evolution, nucleation, and early growth during solidification of aluminum-copper alloys are investigated. In particular, nucleation rates are measured, and contact angles calculated using the classical heterogeneous nucleation theory. Also, growth velocities of the dendrites are measured and compared with those predicted using kinetic models in order to elucidate which model is controlling during recalescence or early growth. Two groups of aluminum alloys with 5.1, 6.6, 15.6, and 30 wt% Cu are studied. One group is uninoculated and the other is grain refined with Ti-B-Al, Ti-B, and Ti-C-Al master alloys. In alloys that exhibit recalescence of primary solid, solidification occurs at a temperature, approximately 0.4 °C or less before the minimum in the curve is reached. Grain refined alloys exhibit dendrites which are equiaxed in morphology and appear as clusters in a matrix of quenched liquid. Uninoculated alloys, however, contain somewhat columnar dendrites. Nucleation rates are composition sensitive and range from 4.8 x 10¹³ m³/s to 1.6 x 10\sp¹⁵ m³/s in uninoculated alloys; with an increase in copper content there is an increase in nucleation rate. In grain refined alloys, the same metric measures from 4.4 x 10¹⁶ m³/s to 1.4 x 10¹⁹ m³/s. The calculated contact angles are consistently smaller in the grain refined alloys when compared with their uninoculated counterparts and are rather sensitive to small changes in the measured undercooling. For example, if the undercooling with respect to the liquidus temperature is known to within ± 1°, the calculated contact angle changes by ± 0.8°. Experimental growth rates in grain refined alloys are less than in uninoculated alloys, because the undercoolings during the early stages of growth are less. While growth rate in uninoculated alloys typically range from 2.2 x 10⁻⁴ m/s in the most concentrated alloy to 1.8 x 10⁻³ m/s in the most dilute alloy, the same metric in grain refined alloys ranges from 7.8 x 10⁻⁵ m/s to 1.7 x 10⁻⁴ m/s. Experimental growth rates compare favorably to dendrite growth models of Ivantsov, and Lipton-Glicksman-Kurz. Consequently, during recalescence and/or early growth, the growth velocity of primary solid is controlled by solute diffusion processes and not by interface kinetics.